1. Field of the Invention
The present invention relates to an image forming apparatus employing a fixing device.
2. Description of Related Art
An electrophotographic printer generally transfers toner corresponding to a print image to a sheet of paper and fixes the toner onto the sheet of paper with application of heat and pressure by a fixing roller and a pressure roller. In the course of a print process, heat of the fixing roller is removed from a surface thereof by a recording medium or the pressure roller. On the other hand, when passage of the recording medium is finished (i.e., the print process is finished), the heat is not removed, causing a rapid increase in surface temperature by emergence of the heat accumulated inside the fixing roller. Such a phenomenon is hereafter referred to as an overshoot. An excess increase in the surface temperature of the fixing roller causes poor fixing quality or damages of a fixing device. Japanese Un-examined Patent Application Publication No. 2001-242741 discloses a method for reducing such an excess increase in the temperature of the fixing roller by rotating the fixing roller for a certain time period while turning off drive of a heater after the print process is finished and passage of the recording medium in the fixing device is completed so as to reduce the overshoot.
In a case where a large number of thick recording media are fixed, however, such a related-art method for reducing the excess increase in temperature by rotation of the fixing roller for the certain time period needs an adequate amount of time to rotate the fixing roller in the post-print process due to an increase of an accumulated heat amount inside of the fixing roller. Consequently, in a case where the rotation of the fixing roller is controlled with respect to each post-print process, there causes a problem of prolongation of a waiting time period until the subsequent print process begins.
It is an object of the present invention to provide an image forming apparatus capable of reducing the overshoot after a rotation of a fixing roller halts while controlling to shorten a rotation time period of the fixing roller, and capable of shortening a time period until the subsequent print process.